Fundamental mode operation in broad area quantum cascade lasers

ABSTRACT

A broad area quantum cascade laser subject to having high order transverse optical modes during operation includes a laser cavity at least partially enclosed by walls, and a perturbation in the laser cavity extending from one or more of the walls. The perturbation may have a shape and a size sufficient to suppress high order transverse optical modes during operation of the broad area quantum cascade laser, where a fundamental transverse optical mode is selected over the high order transverse optical modes. As a result, the fundamental transverse mode operation in broad-area quantum cascade lasers may be regained, when it could not otherwise be without such a perturbation.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The application is a continuation of U.S. patent application Ser. No.15/676,825 filed on Aug. 14, 2017, the entire content of which is herebyincorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government of the United States for all governmentalpurposes without the payment of any royalty.

BACKGROUND

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers that useoptical transitions between electronic sub-bands to produce light. QCLscan be designed to emit in the mid-infrared wavelength range (e.g., 2μm-20 μm), and more recently in the long-infrared wavelength range,e.g., the terahertz spectral range. QCL technology has generally reacheda maturity level where it can provide relatively reliable operation foruse in a large variety of applications. By way of example and notlimitation, detectors that incorporate QCLs can be used for chemicalsensing such as pollution monitoring, gas sensing, medical diagnostics(e.g., through breath analysis), the remote detection of toxic chemicalsand explosives, and others. For applications requiring radiation at asingle frequency, the longitudinal mode selection in QCLs may beprovided, where single longitudinal mode operation of QCLs can beachieved by fabricating the QCLs as distributed feedback lasers(DFB-QCL).

In a typical DFB-QCL device, a grating has grooves etched at the top ofthe device that are aligned perpendicular to the optical axis of thedevice to produce index coupling, which selects the longitudinal modefor single frequency emission. Many embodiments of this basic idea existto improve the selection of the single longitudinal mode in such DFB-QCLdevices.

In contrast to the longitudinal mode, the transverse (or lateral) modein QCLs is generally not selectively controlled in existing devices.Instead, a QCL device may have a sufficiently narrow width such thatonly the fundamental transverse mode is active. The fundamentaltransverse mode ensures that a single diffraction-limited beam along theoptical axis of the laser is emitted, with an angular divergencedetermined by the wavelength of the light and the width of the device.The power level that can be generated in a QCL, as in othersemiconductor lasers, may scale with the area of the device. Ininstances where additional power is desired, a larger area QCL devicemay be fabricated, where a QCL device with a cavity width larger than12-15 micrometers is generally referred to as a broad area device(BA-QCL).

In practice, scaling of the power by enlarging the area of the QCL istypically limited to increasing the cavity length, rather than thecavity width of the QCL. This is because keeping a narrow cavity canmaintain fundamental transverse mode operation, whereas increasing thecavity width may result in the operation of high-order transverse modes,as they become more favorable. For example, if an existing mid-infraredQCL is fabricated with a cavity width of about 15 micrometers, this maylead to the emergence of high-order transverse modes, resulting in modecompetition, beam steering, and loss of brightness. In cavity widths ofabout 20 micrometers and higher, one or several high-order transversemodes may be active where each high-order transverse mode forms aperiodic structure in the near-field of the QCL device, resulting in alaser beam with two distinct lobes, where each lobe deviates from theoptical axis by an angle that becomes larger as the mode numberincreases—see, e.g., Y. Bai et al., APPLIED PHYSICS LETTERS 95, 221104(2009), which is hereby incorporated by reference.

Several approaches have been attempted to produce single lobedemission—the result of a fundamental transverse mode—in BA-QCLs. Theseinclude the use of angled cavities, photonic crystal gratings,gain-guided devices, and the use of a porous structure above the activeregion of the device. However, there are some disadvantages associatedwith these techniques. For example, in angled cavity configuration, thefacet angles and the cavity length must be precisely controlled forsingle lobed emission—see, e.g., D. Heydari et al., APPLIED PHYSICSLETTERS 106, 091105 (2015), which is hereby incorporated by reference.In gain-guided devices, the current spreading determines the effectivewidth of the device, and this results in a strong variation of the beamdivergence with injection current—see, e.g., I. Sergachev et al., OPTICSEXPRESS 24, 19063 (2016), which is hereby incorporated by reference. Inanother approach, lateral constrictions in the waveguide were placedusing a focused ion beam milling technique where only the fundamentalmode was allowed to propagate to produce a Gaussian shaped far-fieldpattern—see, e.g., Bouzi et al., APPL. PHYS. LETT. 102, 122105 (2013),which is hereby incorporated by reference. However, this approach waslimited to devices with a narrow cavity width (w=10 μm), not BA-QCLs,and the trenches had to be filled with metal to provide additionallosses to achieve the desired effect. Therefore, there remains a needfor improved devices, systems, and methods for extracting andmaintaining fundamental transverse mode operation in BA-QCLs.

SUMMARY

In an implementation, a broad area quantum cascade laser subject tohaving high order transverse optical modes during operation includes anoptical cavity at least partially enclosed by walls, and a perturbationin the optical cavity extending from one or more of the walls. Theperturbation may have a shape and a size sufficient to suppress highorder transverse optical modes during operation of the broad areaquantum cascade laser, where a fundamental transverse optical mode isselected over the high order transverse optical modes.

In another implementation, a method includes forming a perturbation inan optical cavity of a broad area quantum cascade laser, where theperturbation extends from one or more walls of the optical cavity. Themethod may also include suppressing, with the perturbation, high ordertransverse optical modes during operation of the broad area quantumcascade laser, where a fundamental transverse optical mode is selectedover the high order transverse optical modes.

In yet another implementation, a broad area quantum cascade laserincludes an optical cavity having an active region disposed between atop cladding and a bottom cladding. The broad area quantum cascade lasermay also include at least two excavations formed in a top surface of theoptical cavity, where the excavations extend into at least the topcladding of the optical cavity. The broad area quantum cascade laser mayalso include a central portion disposed between the excavations, thecentral portion including a top region disposed above the at least twoexcavations. The excavations may be structurally configured to select afundamental transverse mode of light in the optical cavity byconstricting a lateral refractive index profile of the optical cavity.

In another implementation, a method includes forming at least twoexcavations in a top surface of an optical cavity of a broad areaquantum cascade laser, the optical cavity including an active regiondisposed between a top cladding and a bottom cladding, where the atleast two excavations extend into at least the top cladding of theoptical cavity. The method may also include altering a lateralrefractive index profile of the optical cavity.

In yet another implementation, a method includes forming at least twoexcavations in a top surface of an optical cavity of a broad areaquantum cascade laser, the optical cavity including an active regiondisposed between a top cladding and a bottom cladding, the at least twoexcavations extending into at least the top cladding of the opticalcavity, and the at least two excavations structurally configured toselect a fundamental transverse mode of light in the optical cavity byconstricting a lateral refractive index profile of the optical cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings provide visual representations which will beused to more fully describe various representative embodiments and canbe used by those skilled in the art to better understand therepresentative embodiments disclosed and their inherent advantages. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the devices, systems, and methodsdescribed herein. In these drawings, like reference numerals mayidentify corresponding elements.

FIG. 1A is a top view of a representation of a broad area quantumcascade laser, in accordance with the prior art.

FIG. 1B is a front view of a representation of a broad area quantumcascade laser, in accordance with the prior art.

FIG. 2 is a graph showing a far field intensity profile from a broadarea quantum cascade laser where a high order transverse mode isoperational, in accordance with the prior art.

FIG. 3A is a top view of a representation of a broad area quantumcascade laser, in accordance with a representative embodiment.

FIG. 3B is a front view of a representation of a broad area quantumcascade laser, in accordance with a representative embodiment.

FIG. 4 is a top view of a portion of a broad area quantum cascade laser,in accordance with a representative embodiment.

FIG. 5 is a graph showing far field measurement as a function ofexcavation depth, in accordance with representative embodiments.

FIG. 6 is a graph showing current versus voltage, and current versusoutput power, for both an unmodified broad area quantum cascade laser ofthe prior art and a modified broad area quantum cascade laser inaccordance with a representative embodiment.

FIG. 7 is a top view of a portion of a broad area quantum cascade laser,in accordance with a representative embodiment.

FIG. 8 is a graph showing far field profiles of laser beams, inaccordance with representative embodiments.

FIG. 9 is a flow chart of a method for extracting fundamental transversemode operation in a broad area quantum cascade laser, in accordance witha representative embodiment.

DETAILED DESCRIPTION

The various methods, systems, apparatuses, and devices described hereingenerally include extracting and maintaining fundamental transverse modeoperation in broad area quantum cascade lasers.

While this invention is susceptible of being embodied in many differentforms, there is shown in the drawings and will herein be described indetail specific embodiments, with the understanding that the presentdisclosure is to be considered as an example of the principles of theinvention and not intended to limit the invention to the specificembodiments shown and described. In the description below, likereference numerals may be used to describe the same, similar orcorresponding parts in the several views of the drawings.

In this document, relational terms such as first and second, top andbottom, and the like may be used solely to distinguish one entity oraction from another entity or action without necessarily requiring orimplying any actual such relationship or order between such entities oractions. The terms “comprises,” “comprising,” “includes,” “including,”“has,” “having,” or any other variations thereof, are intended to covera non-exclusive inclusion, such that a process, method, article, orapparatus that comprises a list of elements does not include only thoseelements but may include other elements not expressly listed or inherentto such process, method, article, or apparatus. An element preceded by“comprises . . . a” does not, without more constraints, preclude theexistence of additional identical elements in the process, method,article, or apparatus that comprises the element.

Reference throughout this document to “one embodiment,” “certainembodiments,” “an embodiment,” “implementation(s),” “aspect(s),” orsimilar terms means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of such phrases or in various places throughout thisspecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments withoutlimitation.

The term “or” as used herein is to be interpreted as an inclusive ormeaning any one or any combination. Therefore, “A, B or C” means “any ofthe following: A; B; C; A and B; A and C; B and C; A, B and C.” Anexception to this definition will occur only when a combination ofelements, functions, steps or acts are in some way inherently mutuallyexclusive. Also, grammatical conjunctions are intended to express anyand all disjunctive and conjunctive combinations of conjoined clauses,sentences, words, and the like, unless otherwise stated or clear fromthe context. Thus, the term “or” should generally be understood to mean“and/or” and so forth.

All documents mentioned herein are hereby incorporated by reference intheir entirety. References to items in the singular should be understoodto include items in the plural, and vice versa, unless explicitly statedotherwise or clear from the text.

Recitation of ranges of values herein are not intended to be limiting,referring instead individually to any and all values falling within therange, unless otherwise indicated, and each separate value within such arange is incorporated into the specification as if it were individuallyrecited herein. The words “about,” “approximately,” or the like, whenaccompanying a numerical value, are to be construed as indicating adeviation as would be appreciated by one of ordinary skill in the art tooperate satisfactorily for an intended purpose. Ranges of values and/ornumeric values are provided herein as examples only, and do notconstitute a limitation on the scope of the described embodiments. Theuse of any and all examples, or exemplary language (“e.g.,” “such as,”or the like) provided herein, is intended merely to better illuminatethe embodiments and does not pose a limitation on the scope of theembodiments. No language in the specification should be construed asindicating any unclaimed element as essential to the practice of theembodiments.

For simplicity and clarity of illustration, reference numerals may berepeated among the figures to indicate corresponding or analogouselements. Numerous details are set forth to provide an understanding ofthe embodiments described herein. The embodiments may be practicedwithout these details. In other instances, well-known methods,procedures, and components have not been described in detail to avoidobscuring the embodiments described. The description is not to beconsidered as limited to the scope of the embodiments described herein.

In the following description, it is understood that terms such as“first,” “second,” “top,” “bottom,” “up,” “down,” “above,” “below,” andthe like, are words of convenience and are not to be construed aslimiting terms. Also, the terms apparatus and device may be usedinterchangeably in this text.

In general, the devices, systems, and methods described herein mayinclude a broad area quantum cascade laser (BA-QCL) with fundamentaltransverse mode operation, and more specifically to techniques forobtaining and maintaining the fundamental transverse mode operation inBA-QCLs. As such, the disclosure relates generally to semiconductorlight sources, and more particularly, to quantum cascade laser (QCL)devices that emit in the mid-infrared wavelength range (e.g., about 2μm-20 μm), as well as long-infrared wavelength ranges, e.g., theterahertz spectral range. Techniques disclosed herein may include aBA-QCL that sustains fundamental transverse mode operation, e.g., toattain higher brightness in a single beam that is relatively easy tooperate. The devices, systems, and methods disclosed herein may includeany of the techniques described in R. Kaspi et al., “Extractingfundamental transverse mode operation in broad area quantum cascadelasers,” APPLIED PHYSICS LETTERS 109, 211102 (Nov. 23, 2016), which ishereby incorporated by reference.

As discussed above, QCLs are unipolar semiconductor lasers that useoptical transitions between electronic sub-bands to produce light. QCLscan be designed to emit in the mid-infrared and long-infrared wavelengthranges of the electromagnetic spectrum. Unlike typical inter-bandsemiconductor lasers that emit electromagnetic radiation through therecombination of electron-hole pairs across a material band gap, QCLsare unipolar, and laser emission is achieved through the use ofinter-subband transitions in a repeated stack of semiconductor multiplequantum well heterostructures. By way of example, QCLs may be used inthe areas of remote sensing, long-wave imaging, communications, aircraftcountermeasures, and the like.

Broad area lasers generally operate spatially and longitudinallymultimode, and may be used for solid-state laser pumping, sensortechnology, material processing, medical applications (e.g.,photodynamic therapy), as well as other applications known to those ofordinary skill in the art. Broad area laser diodes (which may also bereferred to in the art as broad stripe laser diodes, broad emitter laserdiodes, single-emitter laser diodes, and high power diode lasers) mayinclude edge-emitting laser diodes where the emitting region at thefront facet has the general shape of a broad stripe, with dimensions of,e.g., 1 μm×100 μm.

Thus, QCLs may be the preferred choice for a variety of applications formid-infrared and long-infrared emission from a compact source, due totheir relatively high efficiency at room temperature. Power scaling inQCLs is possible by fabricating broad area devices. However, broad areaQCL devices with cavity widths that exceed approximately 10 μm typicallyexhibit modal instability, non-linear interactions, beam steering, andloss of brightness. When the cavity width is very large, e.g., greaterthan 30 μm, high order transverse modes generally result in a far fieldprofile that is double-lobed, with each lobe propagating at large anglesfrom the optical axis. Often a single or a small number of high ordertransverse modes are operational, because, unlike the typical inter-banddiode laser, filamentation may be naturally suppressed in QCLs.Multi-lobed emission is a large obstacle to producing practical BA-QCLdevices with high brightness.

As such, although fabricating broad area devices may be the simplestmethod to scale the power of QCLs, in the broad area devices, high-ordertransverse modes are operational and may inhibit single beam emissionand reduce brightness. Thus, scaling brightness in QCLs may be severelylimited by the emergence of the high order transverse modes as the lasercavity is made larger. To this end, as discussed herein, the disclosedtechniques may improve upon current technologies that instead result inhigh-order transverse modes. For example, without using techniquesdisclosed herein, if a QCL was fabricated with a cavity width ofapproximately 50 micrometers to increase output power, the ensuing beammay have approximately half the power propagating at an angle of about+35 degrees, and the other half of the power propagating at an angle ofabout −35 degrees from the optical axis of the device. Such anapparently dual beam device may lose half of its potential brightness,and would be relatively difficult to operate as it will not have asingle well controlled emission beam along the optical axis.

Several attempts have been made in the prior art to fabricate a BA-QCLthat sustains fundamental transverse mode operation, and a few aredescribed below by way of example. The disclosed techniques of thepresent teachings may include improvements over each of the followingexamples.

In a first example, a porous structure was incorporated in the cavityalong the entire length of the BA-QCL by electrochemical etching, andthis was reported to suppress higher order transverse modes—see, e.g.,Zhao et al., J. APPL. PHYS. 112, 013111 (2012), which is herebyincorporated by reference. However, a porous structure is not somethingthat can easily be incorporated into a typical QCL fabrication process.

In a second example, a gain guided BA-QCL without distinct sidewalls wasdemonstrated to favor fundamental transverse mode operation because highorder transverse modes cannot be sustained without the sidewalls—see,e.g., Sergachev et al., OPT. EXPRESS 24, 19063 (2016), which is herebyincorporated by reference. However, in this example, due to currentspreading, the divergence angle of the fundamental mode varied with thelevel of current injection, making the laser output characteristicsbecome unpredictable.

In a third example, an angled cavity shape, i.e., a parallelogram withangles that are not 90 degrees, was used as a BA-QCL cavity to filterhigh order transverse modes—see, e.g., Heydari et al., APPL. PHYS. LETT.106, 91105 (2015), which is hereby incorporated by reference. However,in this example, the facet angles and the cavity length must beprecisely controlled in order to produce a single lobed output beam fromthe fundamental mode.

In a fourth example, two short trenches were placed in a narrow (w=10micrometers) cavity QCL device to suppress the emergence of the higherorder transverse mode—see, e.g., Bouzi et al., APPL. PHYS. LETT. 102,122105 (2013), which is hereby incorporated by reference. However, asdiscussed above, it was concluded that high order mode suppression onlyoccurred when the trenches were filled with a metal to providesufficient losses to the high order mode, and no BA-QCL cavity widthswere explored.

In a fifth example, BA-QCL devices emitting in the terahertz wavelengthrange were reported to have high order transverse modes suppression whenplasmon layers at the edges of the top metal cladding were exposed toprovide losses to the high order transverse modes—see, e.g., Fan et al.,APPL. PHYS. LETT. 92, 031106 (2008), which is hereby incorporated byreference. However, the plasmon layers are not incorporated into QCLdevices emitting at mid-infrared wavelengths.

Thus, as demonstrated by the above examples, BA-QCLs that extract andmaintain fundamental transverse mode operation are desirable,particularly if compatible with mass-fabrication methods. Devices,systems, and methods described herein may be used to this end. Morespecifically, devices, systems, and methods described herein maysuppress high-order transverse modes by forming excavations into thelaser cavity that change the lateral index profile in a BA-QCL. In otherwords, in certain implementations, a BA-QCL may be modified by placing alocal perturbation in the lateral refractive index profile in a mannerthat selectively favors lasing in the fundamental transverse mode, whenit would not otherwise be favored. These techniques may be translatableto all commonly practiced semiconductor fabrication methods, and mayresult in the scaling of brightness in BA-QCLs. As a result, in certainimplementations described herein, fundamental transverse mode operationin BA-QCLs can be restored, recovering single beam emission withrelatively high brightness thus making them more useful in manyapplications.

Stated otherwise, because power scaling in BA-QCLs results in theoperation of high order transverse modes with a far-field profileincluding two lobes propagating at large angles relative to the opticalaxis, disclosed herein are techniques for suppressing the high ordertransverse modes that can extract the fundamental transverse mode andprovide emission along the optical axis. By generating localized changesin the refractive index within the waveguide in the form of shortexcavations, other excavations, trenches, or the like (e.g., formed by afocused ion beam milling technique), broad area devices may be providedwhere most of the power is contained in a near diffraction-limited beamthat provides relatively high brightness.

Thus, implementations may include a BA-QCL in which the fundamentaltransverse mode operation can be sustained even when the cavity width isenlarged to produce higher power. Specifically, implementations mayinclude a localized protrusion into the optical cavity (e.g., by anexcavation formed therein, or otherwise a perturbation of the opticalcavity) to provide a change in the lateral index profile of a BA-QCL.The precise shape and placement of an excavation of the optical cavitymay provide preferential losses to the high-order transverse modes infavor of the fundamental mode.

As described herein, an index change that produces fundamentaltransverse mode operation may be provided by at least two excavationsformed into the optical cavity, e.g., the top surface thereof. Theexcavations into the optical cavity may be formed by the deliberateetching of trenches in the device using an ion beam milling tool or anyother tool capable of forming such excavations. The excavations may aidin providing a lateral refractive index profile that attenuateshigh-order transverse modes. Thus, in implementations, the net resultmay be the extraction of the fundamental mode, when it would nototherwise operate.

When the excavations are positioned in a predetermined and appropriatemanner, e.g., they have the appropriate width, length, and depth, theexcavations may modify the threshold behavior of the high-order modessuch that operation of the fundamental mode is once again more favorabledespite the enlarged cavity width of the BA-QCL device. It should benoted that, in certain implementations, the lateral index constrictionis not an optical “aperture” because it does not block light. Instead,the fundamental mode becomes the most favorable mode because it is theleast affected, while extending to the full width of the BA-QCL device.The resulting laser beam from the BA-QCL may be single lobed, alignedwith the optical axis, and provide enhanced brightness. The disclosedtechniques may be valid for any emission wavelength, and any method bywhich the excavations as described herein can be formed.

For context, FIG. 1A is a top view of a representation of a BA-QCL 100in accordance with the prior art, and FIG. 1B is a front view of theBA-QCL 100. More specifically, FIG. 1B is a sectional view throughSection A-A of FIG. 1A, where both figures show a BA-QCL 100 accordingto the prior art, e.g., without any perturbation of an optical cavity110 of the BA-QCL 100.

FIGS. 1A and 1B show the optical cavity 110, which may approximate auniform box whose geometry is defined by the cavity width 112, thecavity length 114, and sidewalls 116 that are etched to a depth below anactive region 120. When the sidewalls 116 are metal terminated, whichmay be a typical fabrication method of BA-QCLs 100, the near field ofthe transverse mode in the optical cavity 110 may be described as asinusoidal function, i.e., the solution to the Hemholtz equation, asshown by the sinusoidal curve 130 in FIG. 1B. Each of these sinusoidalfunctions may have a near field amplitude A(z), written asA(z)=sin(Nπz/w), where N is the transverse mode number and w is thecavity width 112. Each of these sinusoidal functions in the near fieldmay, in turn, produce a far-field intensity distribution in which lobeswill appear at angles±Θ, where Θ=arcsin(λN/2w), where λ, is thewavelength.

Among the many sinusoidal functions defined by N=1,2,3, and so on, aparticular one may have the most advantageous gain versus losscharacteristics, and will become operational. When the cavity width 112is relatively small (e.g., w<10 micrometers), then the most advantageousmode may be N=1, otherwise referred to as the TM00, or the fundamentalmode. That is, the angle±Θ may be near zero degrees from the opticalaxis 102 of the laser, thus forming a single beam.

FIG. 2 is a graph 200 showing a far field intensity profile 202, e.g.,for the BA-QCL 100 shown in FIGS. 1A and 1B, i.e., where a high ordertransverse mode is operational. When the width of the optical cavity 110is relatively large (e.g. w≈90 micrometers), as in a BA-QCL 100, themost advantageous transverse mode may have a large N, and two lobes 204,206 in the far field will be centered at +Θ and −Θ degrees from theoptical axis 102, as exemplified by the data shown in FIG. 2. In thisexample, the far field intensity profile 202 from a BA-QCL 100 withw=100 micrometers and A 4.85 μm is shown. The two lobes 204, 206 arecentered at about ±38 degrees, corresponding to a transverse mode numberN=23. Such a high order transverse mode is schematically depicted as ageneric sinusoidal curve 130 in FIG. 1B.

By way of analogy, the various transverse modes N=1,2,3, and so on, canbe thought of as being in competition with each other. If all modes wereequally competitive, then perhaps they could all co-exist. In certainlasers, if not equally competitive, many of these transverse modes inBA-QCLs are nearly competitive. In other words, there may only be asmall preference for a particular high order transverse mode over theothers. Thus, the dependence of threshold gain versus mode number may bequite shallow. In this manner, occasionally two or three differenttransverse modes that are nearly-competitive may co-exist in the sameBA-QCL.

In the disclosed techniques, a small perturbation in the lateral indexprofile within an optical cavity of a BA-QCL, if placed correctly, maydrastically change the mode selection behavior in the BA-QCL in favor ofthe fundamental mode. As discussed herein, this may be accomplished byplacing at least two excavations (e.g., at least two nominally identicalrectangular trenches) in the top of the device. These excavations may belocated along the edges of the device, such that the center of thedevice is not disturbed. This is schematically shown in FIG. 3A, whichis described in more detail below.

FIG. 3A is a top view of a representation of a BA-QCL 300, and FIG. 3Bis a front view of a representation of a BA-QCL 300, in accordance witha representative embodiment. More specifically, FIG. 3B is a sectionalview through Section B-B of FIG. 3A, where both figures show a BA-QCL300 according to a representative embodiment.

The BA-QCL 300 may include an optical cavity 310, which for the sake ofexample may approximate a substantially uniform box whose geometry isdefined by the cavity width 312, the cavity length 314, and sidewalls316. It should be understood that FIG. 3A and FIG. 3B are notnecessarily drawn to scale, and are provided by way of representation.It will be understood that the optical cavity 310 may be otherwisereferred to herein as a “laser cavity” or the like. The optical cavity310 may include an active region 320 disposed between a top cladding 318and a bottom cladding 322. Because the device may include a BA-QCL 300,the cavity width 312 of the optical cavity 310 may be greater than orequal to 10 μm. Other dimensions for the optical cavity 310 are also orinstead possible.

The BA-QCL 300 may include at least two excavations 330 formed in a topsurface 311 of the optical cavity 310. The excavations 330 may extendinto at least the top cladding 318 of the optical cavity 310. Theexcavations 330 may be structurally configured to select a fundamentaltransverse mode 324 of light in the optical cavity 310 by constricting alateral refractive index profile 304 of the optical cavity 310. Incertain implementations, the fundamental transverse mode 324 of thelight in the optical cavity 310 would otherwise not be favored withoutthe excavations 330. Thus, in certain implementations, without theexcavations 330, high order transverse modes of the light would bepresent in the optical cavity 310. As such, the excavations 330 may bestructurally configured to suppress the high order transverse modes ofthe light in the optical cavity 310. In this manner, the excavations 330may be structurally configured to provide a loss to the high ordertransverse modes of the light.

The excavations 330 may include a depth 332, which may be predeterminedto achieve certain properties or characteristics of light in the opticalcavity 310 or emitted by the BA-QCL 300. The depth 332 may be consistentfor all excavations 330 in a BA-QCL 300, or it may vary from one or moreother excavations 330 in the same BA-QCL 300. Similarly, the depth 332of the excavations 330 may be consistent across a plurality of BA-QCLs300, or it may vary among BA-QCLs 300. In certain implementations, oneor more of the excavations 330 extend beyond the top cladding 318 andinto the active region 320. The excavations 330 may also or insteadextend into the bottom cladding 322. Instead, the excavations 330 may berelatively shallow, extending into only the top cladding 318. In animplementation, the depth 332 of the excavations 330 is about 4.5 μm.The excavations 330 may also or instead extend into the optical cavity310 from the bottom of the optical cavity 310, starting with the bottomcladding 322. The excavations may also or instead extend into theoptical cavity 310 from its sidewalls 316.

The excavations 330 may include a size and shape that is consistent forall excavations 330 in a BA-QCL 300, or it may vary from one or moreother excavations 330 in the same BA-QCL 300. Similarly, the size andshape of the excavations 330 may be consistent across a plurality ofBA-QCLs 300, or it may vary among BA-QCLs 300. In certainimplementations, at least one of a size, a shape, and a depth 332 of oneor more of the excavations 330 is selected to alter the lateralrefractive index profile 304 of the optical cavity 310 to affect modeselection that is specific to the light generated by the BA-QCL 300. Theshape of one or more of the excavations 330 when viewed from above theoptical cavity 310 may include a polygon. For example, as shown in FIG.3A, the shape of one or more of the excavations 330 when viewed fromabove the optical cavity 310 may include a rectangle. Other shapes arealso or instead possible for the excavations 330. By way of example, ashape of one or more of the excavations 330 when viewed from above theoptical cavity 310 may include a curve. In this manner, the shape of oneor more of the excavations 330 may include at least one of a circle, anellipse, and an oblong shape. The effect of the geometry of theexcavations 330 of the BA-QCL 300 is discussed in more detail below.

The BA-QCL 300 may include a central portion 340 disposed between theexcavations 330. The central portion 340 may include a top region 342disposed above the two excavations. In other words, the central portion340 may form a plateau disposed between the excavations 330 that isdisposed at a height along a z-axis 307 that is above the bottom of theexcavations 330 defined by the depth 332. A longitudinal centerline 301of the optical cavity 310 may be disposed through the central portion340 between the excavations 330. As shown in FIG. 3A, the longitudinalcenterline 301 may be disposed along an optical axis 302 of the opticalcavity 310 when viewed from above.

The light beam outside of the optical cavity 310 may include one or morelobes of intensity propagating in different directions that aregenerated by the BA-QCL 300. Thus, in certain implementations, theexcavations 330 may be structurally configured to provide primarily asingle-lobed laser beam 350 outside of the optical cavity 310. Thesingle-lobed laser beam 350 may be substantially aligned to the opticalaxis 302 of the optical cavity 310. In certain implementations, thesingle-lobed laser beam 350 is relatively brighter than a counterpartlaser beam would be if the counterpart laser beam was passed through asimilar optical cavity that lacks the excavations 330. Thus, theexcavations 330 may create a brighter single-lobed laser beam 350 thanan optical cavity lacking such excavations 330. Also, in this manner,the light in the optical cavity 310 may include a mid-range infraredwavelength. For example, the light may include a wavelength in a rangeof 2 μm to about 20 μm. The light in the optical cavity 310 may also orinstead include a long-wave infrared wavelength. For example, the lightmay include a wavelength of at least 1 terahertz. Other wavelengths arealso or instead possible for the light in the optical cavity 310.

Some other characteristics of the excavations 330 will now be described.

The excavations 330 may be structurally configured to achieve a higherbrightness of the light (e.g., the single-lobed laser beam 350) outsideof the optical cavity 310. In this manner, the excavations 330 may bestructurally configured to improve the beam-quality of the emitted lightto achieve higher brightness of a laser beam outside of the opticalcavity 310.

As discussed above, the excavations 330 may be different from anaperture. In other words, the excavations 330 may not block light frompassing therethrough. Instead, the BA-QCL 300 may further include anoptical aperture 360 downstream of the optical cavity 310. Thus, theconstriction of the lateral refractive index profile 304 provided by theexcavations 330 may not be the same as providing an optical aperture360. Instead, the resulting beam divergence as a result of placing theexcavations 330 may approach a near diffraction-limited beam originatingfrom the fundamental mode occupying the entire width of the BA-QCL 300.

One or more of the optical cavity 310 and the excavations 330 may lackmetal. For example, the excavations 330 may define voids, where thevoids lack any additional material after they are formed. In thismanner, the excavations 330 may not be refilled with any material afterformation thereof. In other implementations, the excavations 330 arerefilled with material after formation thereof. Thus, the excavations330 do not have to remain unfilled. If filled with a material such as ametal, an insulator, or oxides, the disclosed techniques may not changebecause the lateral index profile should continue to provide benefits asdisclosed herein. Thus, changing the refractive index within theexcavations 330 may only be dependent upon the geometry of theexcavations 330, which can be optimized accordingly.

By way of example, in an implementation where one or more of theexcavations 330 are substantially shaped as rectangles such as thatshown in FIG. 3A, a length 334 of an excavation 330 may be about 150 μm,and a width 336 of the excavation 330 may be about 30 μm. In such animplementation, or in other implementations, the central portion 340disposed between the excavations 330 is about 24 μm wide. Therectangular or box-shaped excavations 330 may be defined by their length334, width 336, and depth 332. It will be understood that in thisexample embodiment (i.e., the embodiment shown in FIG. 3A), the numberof the excavations 330 and the dimensions of the excavations 330 areprovided for illustrative purposes only and not by way of limitation. InFIG. 3A, two excavations 330 are shown placed on the BA-QCL 300, whichmay have a cavity width 312 of about 90 micrometers and a cavity length314 of about 3 millimeters, e.g., the same as the unperturbed devicethat produced the data in the graph 200 of FIG. 2. In this exampleembodiment of FIG. 3A, and as discussed above, the length 334 of theexcavations 330 may be about 5% of the cavity length 314 of the BA-QCL300.

The alignment of the excavations 330 may be selected such that theexcavations 330 perform various functions in the optical cavity 310 ofthe BA-QCL 300 as described herein. In an implementation, theexcavations 330 are aligned substantially parallel relative to oneanother. For example, the excavations 330 may be aligned substantiallyparallel to an optical axis 302 of the optical cavity 310. Theexcavations 330 may instead be substantially aligned along an axissubstantially perpendicular to an optical axis 302 of the optical cavity310, e.g., the x-axis 303. The excavations 330 may be disposed at thesame location along the cavity length 314, or one or more of theexcavations 330 may be disposed at different locations along a length ofthe optical cavity 310, e.g., along the y-axis 305.

As discussed above, the excavations 330 may be formed by using an ionbeam milling tool or any other tool capable performing a deliberateetching, ablation, cutting, or removal of material, e.g.,photolithographic etching or plasma etching, ion implantation, selectivecurrent injection, and so on. One of ordinary skill will recognize thatother manufacturing tools and techniques may be used to create theexcavations 330, and any of which may be used in conjunction with thedevices, systems, and methods described herein. Therefore, the formationof the excavations 330 can be accomplished by a variety of techniquesthat include standard photolithographic etching methods, which areroutinely incorporated into the mass fabrication methods for QCLdevices. The method or tool used to form the excavations 330 may notaffect the results, e.g., the experimental example results discussedherein.

The number of the excavations 330 included in the optical cavity 310 maybe selected such that the excavations 330 extract certain performance ofthe light in the optical cavity 310 of the BA-QCL 300 as describedherein. In certain implementations, the optical cavity 310 may includeat least two excavations 330, at least four excavations 330, at leasteight excavations 330, and so on. The excavations 330 may be split ingroupings or pairs, e.g., on opposite sides of the central portion 340or a plane intersecting the central portion 340, e.g., on opposite sidesof a plane substantially disposed along the y-axis 305 and the z-axis307. Other numbers for the excavations 330 are possible, including anodd number of excavations 330. Also, in an implementation, only a singleexcavation 330 is present.

In certain implementations, each excavation 330 in the optical cavity310 is substantially identical. In other implementations, one or more ofthe excavations 330 are different, e.g., including at least one of adifferent size, shape, or depth 332 as one or more other excavations 330in the optical cavity 310.

Thus, in certain implementations, the BA-QCL 300 of FIGS. 3A and 3B, andas otherwise described herein, may be subject to having high ordertransverse optical modes during operation, where the BA-QCL 300 includesa laser cavity enclosed by walls (e.g., the optical cavity 310 andsidewalls 316 shown in the figure). The BA-QCL 300 may further include aperturbation in the laser cavity extending from one or more of thewalls, where the perturbation has a shape and a size sufficient tosuppress high order transverse optical modes during operation of theBA-QCL 300, whereby a fundamental transverse optical mode is selectedover the high order transverse optical modes.

The perturbation described directly above may be in the form of one ormore excavations 330 as described herein, e.g., a plurality ofexcavations 330. However, in general, the perturbation may include anymodification to the laser cavity that acts to suppress high ordertransverse optical modes during operation of the BA-QCL 300. In thismanner, the perturbation may include a change to the laser cavityincluding without limitation one or more of a change in material, aphysical change, a mechanical property change, an electrical propertychange, a radiation change, a chemical change, and the like. It shouldalso be noted that, depending upon perspective, the perturbation may bethought of as a protrusion, e.g., when an excavation 330 is viewed fromthe perspective of an interior of the optical cavity 310, and theexcavation 330 protrudes into the interior of the optical cavity 310.Thus, in certain implementations, the perturbation includes one or moreexcavations 330, but other forms of perturbations that modify the lasercavity to suppress high order transverse optical modes during operationof the BA-QCL 300 are also or instead possible.

As discussed herein, the laser cavity may have a lateral refractiveindex profile 304, where one or more of the shape and the size of theperturbation is selected to modify the lateral refractive index profile304. The laser cavity may include an active region 320 structurallyconfigured to produce photons, where the perturbation extends into theactive region 320.

As discussed above, the perturbation may be in the form of a pluralityof excavations 330. In certain implementations, each of the plurality ofexcavations 330 includes an identical shape and size. Each of theplurality of excavations 330 may include a shape and a size selected sothat the plurality of excavations 330 collectively modify the lateralrefractive index profile 304 of the laser cavity.

As discussed herein, the laser cavity may include an optical axis 302.In certain implementations, the plurality of excavations 330 arestructurally configured to collectively select the fundamentaltransverse optical mode over high order transverse optical modes,whereby the BA-QCL 300 emits a laser beam having a single lobe alignedwith the optical axis 302 (i.e., the single-lobed laser beam 350 shownin the figure). In certain implementations, the high order transverseoptical modes and the fundamental transverse optical mode are orthogonalto the optical axis 302. A first plane may intersect the optical axis302, e.g., a first plane 303 that is disposed along the x-y plane, i.e.,a plane disposed along both the x-axis 303 and the y-axis 305 shown inthe figure. The excavations 330 may be disposed along the first plane(e.g., the x-y plane), or the first plane may otherwise intersect atleast a portion of one or more of the excavations 330. A second plane(also intersecting the optical axis 302) may be disposed orthogonal tothe first plane. The second plane may be disposed along the y-z plane,i.e., a plane disposed along both the y-axis 305 and the z-axis 307shown in the figures. Thus, an edge of the second plane could berepresented by the centerline 301 shown in FIG. 3B. In certainimplementations, an equal number of the plurality of excavations 330 isdisposed on either side of the second plane. For example, the pluralityof excavations 330 may include a pair of excavations 330, where a gap isdisposed between a first excavation in the pair of excavations 330 and asecond excavation in the pair of excavations 330. The gap may be definedby the central portion 340 that is shown in the figures. Thus, in animplementation, the second plane intersects the optical axis 302 andbifurcates the gap, where the first excavation is disposed on one sideof the second plane at a first distance from the second plane, and thesecond excavation is located on a second side of the second plane, alsoat the first distance from the second plane 305. Other configurationsare also or instead possible, such as where the distance from the secondplane is not equal for one or more of the excavations 330.

FIG. 4 is a top view of a portion of a BA-QCL, in accordance with arepresentative embodiment. More specifically, FIG. 4 is an image takenby an electron microscope of a top view of an optical cavity 410 withtwo (substantially rectangular) excavations 430 formed on a top surface411 thereof. For context and perspective, also shown in FIG. 4 are thecenterline 401 (which may be disposed along the optical axis), thex-axis 403, and the y-axis 405. The BA-QCL device shown in the figurewas fabricated using a double-channel scheme where two channels 406 areetched below the active region to define the lateral sidewalls of acavity with cleaved facets. In the example embodiment shown in FIG. 4, afocused ion beam (FIB) tool directed a 5 kV Ga ion beam onto the surfaceof the BA-QCL device to remove material from the designated area in theshape of the substantially rectangular excavations 430.

Presented herein by way of example with reference to FIGS. 5-8, is asystematic empirical study of how the geometry of the excavations canaffect the far field pattern, as well as the threshold and powercharacteristics of the BA-QCL device. In these examples, the QCL devicesunder study were fabricated from structure grown by gas source molecularbeam epitaxy on an n− InP(001) substrate. The strain compensated activeregion was designed to emit near 4.85 μm, and included 30 stages withmultilayer GaInAs/AlInAs injector regions in each stage. The InP topclad included an approximately 3 μm thick layer with the carrierconcentration rising from about 10¹⁶/cm³ to 10¹⁷/cm³, and an additionalapproximately 1 μm thick layer of n+ InP with 10¹⁹/cm³. The BA-QCLdevices were fabricated using a double-channel scheme where tworelatively deep channels were etched below the active region that definethe lateral sidewalls of an otherwise uniform cavity with cleavedfacets. After depositing the insulating layers and the contact metal, anadditional approximately 5 μm thick layer of gold electroplating wasdeposited, except in a small section near the facets of the device toallow for ease of facet cleaving. The devices were mounted without facetcoatings, in the epi-up configuration, and tested at room temperature inthe pulsed regime using 500 ns pulses with a duty cycle of 0.5% tominimize heating. In the examples, the peak power may be estimated bymeasuring the average power using a calibrated thermopile detector andmultiplying by 200 to reflect the duty cycle. Far field measurementswere conducted with a point detector mounted on a motorized rotatingpivot arm about 30 cm away from the device.

In the example experiment shown in FIGS. 5 and 6, the effect of thedepth of the excavations 430 (e.g., in the optical cavity 410 of theBA-QCL of FIG. 4) on the transverse mode behavior was explored. It willbe understood that the ideal geometry of the excavations 430 may befound by optimizing the length as well as the width and the depth of theexcavations 430 with respect to the width of the optical cavity 410 ofthe BA-QCL, such that the desired balance between suppressing high ordermodes while minimizing the losses to the fundamental mode can beachieved. The example excavation depth study conducted in FIGS. 5 and 6may provide empirical data that can be helpful to future calculations ofmode behavior based on the strength of index coupling.

As shown in FIG. 4, in this example excavation depth study, a singlepair of substantially rectangular excavations 430, about 150 μm inlength separated by about 24 μm, were included on a BA-QCL device with awidth of about 90 μm. For obtaining the data of FIGS. 5 and 6, theexcavations 430 were progressively milled after each measurement of thefar field spectrum. The image in FIG. 4 shows the placement of theexcavations 430 relative to the back facet of the BA-QCL device. Farfield spectra collected after each incremental removal of material isshown in FIG. 5. For clarity, the background level of each spectrum isplaced at the y-axis value on the graph 500 indicating the estimateddepth of that particular excavation 430.

Thus, FIG. 5 is a graph 500 showing far field measurement as a functionof excavation depth, in accordance with a representative embodiment. Inthe graph 500, for the purpose of illustration, the depth of theexcavations (labeled as “Trench depth (micrometers)” on the graph 500)is varied to show examples of the effect on high order transverse modeselection in a BA-QCL, e.g., a BA-QCL having an optical cavity 410 asshown in FIG. 4. In these examples, after each new depth, the BA-QCLdevice was re-tested in an identical manner, and the far field angle wasmeasured, where the far field angle measurement as a function of thedepth of the excavations is shown in the FIG. 5.

As shown in FIG. 5, initially, the BA-QCL device without anymodifications exhibits two distinct pairs of lobes in the first farfield 501. This demonstrates that two high order transverse modes arevery nearly competitive with each other, and co-exist. Once theexcavations are formed, it is apparent from the figure that the locallateral index constriction may be capable of influencing high ordertransverse mode selection, with lower mode numbers N being favored asthe excavations become deeper. Specifically, this is demonstrated in theexample by the first far field 501, the second far field 502, the thirdfar field 503, and the fourth far field 504 shown in the graph 500.

Therefore, as shown in FIG. 5, prior to creation of the excavations 430,the device exhibited two distinct transverse modes that wereoperational. As the depth was increased, a gradual suppression of highorder transverse modes in favor of the fundamental mode was observed.Specifically, it was observed that between 0 and about 2.5 μm of depth,the excavations 430 had a relatively small effect on mode selection,primarily changing the relative intensity of the two transverse modes infavor of the mode with the lower mode number and smaller angle. Atdepths of about 3 and about 3.5 μm, the far field spectra indicated thepresence of a larger number of transverse modes that become competitiveas the higher order modes were suppressed. As the excavations 430 reachinto the active region, the emergence of the fundamental mode where themajority of the laser power is contained was observed. The optimaldepth, in this example experimental case, was observed to be near 4.5 μmfrom the surface, which was well inside the active region. This geometryrepresented a point where the losses were most selectively induced onthe higher order modes, while the effect on the fundamental moderemained small. It is noted that, in the example experiment, even inthis case, the M=2 mode was still evident. Finally, when the depth wasextended even further from the surface (e.g., about 5.4 μm), well nearthe bottom of the active region, a return to multi high-order modeoperation was observed. This geometry is presumably where high ordermodes residing only along the unconstrained length of the cavity arepreferred.

In summary, in this example embodiment, the optimal depth corresponds toan excavation depth approximately half way down the active region,nearly 4.5 micrometers form the top surface. At this excavation depth,the BA-QCL device was observed to emit primarily with the fundamentaltransverse mode as shown in the fourth far field 504. Any additionaldepth increases may degrade the laser in the BA-QCL device.

The results shown in FIG. 5 can be explained by the selective nature ofthe localized index profile in this example embodiment. The lateralindex profile has a similar shape to the lateral physical profile, e.g.,shown as the cross-section profile at Section B-B in FIGS. 3A and 3B.The high refractive index of the BA-QCL device material was also locallyreduced wherever material is removed. Thus, the resulting index profilemay favor transverse modes that have the least overlap with theperturbations, such as the fundamental mode that has an intensitydistribution that is only peaked in the center, and disfavors modes thathave higher overlap with the perturbations such as the higher ordertransverse modes.

The specifics of the unaltered BA-QCL device, and the specific geometryof the excavations, may together determine the selection of transversemodes. A salient feature of the disclosed devices, systems, and methodsmay be that any excavation geometry can be successful if it can providethe selectivity to allow the operation of the fundamental transversemode in the BA-QCL device. Thus, in implementations, a variety ofgeometries are possible for the excavations as described herein.

FIG. 6 is a graph 600 showing current versus voltage (represented by afirst set 601 of curves), and current versus output power (representedby a second set 602 of curves), for both an unmodified BA-QCL 603 (e.g.,of the prior art) and a modified BA-QCL 604 (e.g., in accordance with arepresentative embodiment). Specifically, FIG. 6 shows lasercharacterization curves (the first set 601 and the second set 602 ofcurves) comparing an unmodified BA-QCL 603 (i.e., before a perturbationis placed in the laser cavity—e.g., the excavations as explained herein)to the same BA-QCL after a perturbation is placed in the laser cavity(i.e., the modified BA-QCL 604). Thus, the first set 601 of curves showthe current through the BA-QCL as a function of the voltage that isprovided to get that current, and the second set 602 of curves shows thetotal power emitted from the BA-QCL as a function of the current throughthe BA-QCL, where the threshold current is the minimum current beforegetting any light from the BA-QCL, and the slope efficiency is the slopeof the power versus current curve. As explained in more detail below,the graph 600 demonstrates that a BA-QCL with the perturbation has aslightly higher threshold current, and a slightly lower slopeefficiency, but the resulting drop in emitted power is only about 10% atthe maximum current shown, which is not relatively large.

For the data shown in the graph 600, which is provided by way ofexample, a BA-QCL device with w≈90 μm was mounted uncoated in an epi-upconfiguration for testing at room temperature using 500 ns pulses at0.5% duty cycle. After initial characterization, two rectangularexcavations were milled to a depth of about 4.5 μm in a geometry similarto that shown in FIG. 4. A comparison of power-current-voltage (LIV)characteristics are shown in FIG. 6, where a relatively small<10%increase in the threshold current, and a minimal change in the slopeefficiency, were observed in this example experiment. It should be notedthat the far field spectra showed no discernible change in thetransverse mode structure as a function of injection current, and morethan 95% of the total power was contained within a divergence angle of±10 degrees from the optical axis. With only about a 13% drop in powerat the maximum current tested, the modified BA-QCL 604 was able to reachhigh power with high brightness in a usable form, which was notpreviously available in the unmodified BA-QCL 602.

There may be two concerns when altering the mode behavior in a BA-QCL ina manner disclosed herein, but each of these concerns can be amelioratedas discussed below. The first concern may include whether the alterationcauses the BA-QCL to lose power or some other feature that will make itsubstantially less desirable. As shown in FIG. 6, the exampleexperimental results indicate that the modified BA-QCL 604 may exhibitan approximately 10% increase in threshold current, and an approximately13% drop in output power at the same injection current. However, due tothe operation of the fundamental transverse mode, and the ensuingimprovement in beam quality, the brightness of the modified BA-QCL 604may be increased by approximately four-fold, thus alleviating thisconcern.

A second concern when altering the mode behavior in a BA-QCL in themanner disclosed herein may include whether the benefits are toosensitively affected by the excavation geometry, and that the properexcavation geometry may be difficult to duplicate. To better understandthis sensitivity, an example embodiment of how the excavation geometryeffects mode selection is provided below.

By way of example, the far field emission profile from the same BA-QCLdevice is compared after an additional pair of narrow excavations areplaced adjacent to the previous pair, where each excavation hassubstantially the same nominal depth. FIG. 7 is a top view of a portionof a BA-QCL, in accordance with a representative embodiment. Morespecifically, FIG. 7 is an image taken by an electron microscope of atop view of an optical cavity 710 with pairs of excavations 730 formedon a top surface 711 thereof, i.e., after placing four pairs ofexcavations 730 in the example experiment being discussed. In thisexample, each individual excavation 730 is approximately 5 micrometerswide and 150 micrometers long. After four pairs of excavations 730 areplaced in the optical cavity 710, the central portion 740 isapproximately 24 micrometers wide.

In the example shown in FIGS. 7 and 8, the experiment was conducted on adevice with w≈90 μm and a nominal cavity length of 3 mm. After initialcharacterization, four pairs of excavations 730 were etched near theback facet, one pair at a time, starting from the outer edge of thedevice. Each trench was approximately 5 μm wide, 150 μm long, and 4.5 μmdeep, completely contained in the region without a thick golddeposition. After the fourth pair, the unaffected area (i.e., thecentral portion 740) between the excavations 730 was approximately 24μm. In FIG. 8, the far field spectra collected after etching each pairof excavations 730 are shown in this example experiment.

FIG. 8 is a graph 800 showing far field profiles of laser beams, inaccordance with representative embodiments. Specifically, FIG. 8 showsthe far field profile of a laser beam before and after each of the pairsof excavations 730 are placed in the optical cavity 710 of FIG. 7. Thatis, FIG. 8 shows: an original far field profile 805 representing anoriginal, unaltered BA-QCL device; a first far field profile 801representing a single pair of excavations; a second far field profile802 representing two pairs of excavations; a third far field profile 803representing three pairs of excavations; and a fourth far field profile804 representing four pairs of excavations. As shown in the original farfield profile 805 of the graph 800, in the original unaltered BA-QCLdevice, only a high order transverse mode 806 is active. As shown in thesecond far field profile 802 of the graph 800, after two pairs ofexcavations are placed in the optical cavity, the fundamental mode 808co-exists with a selection of other higher order transverse modes 806.As shown in the third far field profile 803 of the graph 800, after thethird pair of excavations is placed in the optical cavity, most of thepower is emitted in the fundamental mode 808. As shown in the fourth farfield profile 804 of the graph 800, the fourth pair of excavationsproduces a relatively cleaner, single peak profile 810.

This particular example embodiment, and the resulting data, illustratethat while the mode selection is affected by the geometry, thesensitivity may not be not very acute. This suggests that excavationswith many other shapes or depths can be formed to provide a level ofmode selectivity that will satisfy a BA-QCL designer, which canalleviate the second concern discussed above. For example, excavationsthat are shallower, but longer, may provide a similar effect. Therefore,only a small perturbation in the optical cavity may have beneficialeffects.

Thus, the results shown in FIGS. 7 and 8 indicate a very stronginfluence of the excavations 730 on the transverse mode selection in thedevice, where the lateral constrictions provide preferential losses tothe higher order modes. Without the excavations 730, the device exhibitstwo lobes at ±38 degrees that is indicative of a single stable highorder transverse mode described by sin Θ=(λM)/(2w), where mode numberM=23, Θ is the emission angle of the lobes, and λ is the wavelength. Inthis example experiment, the first pair of excavations 730 is sufficientto disrupt this transverse mode, resulting in a selection of lower ordermodes that are simultaneously oscillating, with no clear dominant mode.This suggests that the various transverse modes are nearly competitivewith each other, and can be rather easily influenced by small geometricdisruptions in the optical cavity 710. In this example experiment, onlytwo pairs of excavations 730 are able to extract the fundamental mode.The additional pairs of excavations 730 may help suppress most othertransverse modes such that the power is primarily contained in thecentral lobe as seen in FIG. 8.

The angular half-width of the central lobe from the w≈90 μm device maybe approximately 47 mrad, giving a beam parameter product (BPP) of about2.1 mm*mrad. For comparison, a diffraction limited Gaussian beam fromthis device lasing at λ≈4.85 μm should have a BPP of 1.54 mm*mrad.Measurement errors notwithstanding, this suggests that a beam that isbetter than 1.5 times the diffraction limit in the transverse directionmay be achieved. This may also imply that the fundamental mode extendsacross the entire width of the BA-QCL cavity.

For a more complete empirical optimization of the geometry ofexcavations, considerations may include the length of the excavations,their shape, and their position along the BA-QCL cavity. Given that theBA-QCL cavity width, cavity length, wavelength, and gain spectrum mayalso play a role, rigorous modeling of the perturbation to each modewill, in principle, directly guide a BA-QCL designer to an optimizedgeometry. Sufficient information may also or instead be available tofabricate a nearly-optimized BA-QCL device by a direct comparison ofoperational characteristics.

Mathematical modeling may thus be used to make predictions to fine tunethe geometry of the excavations for greater effect. While such modelingis not discussed in detail herein, a BA-QCL designer may apply thedisclosed techniques regardless of the wavelength, geometry, and qualityof an unaltered BA-QCL device. Thus, while a predictive model that tiesthe geometry of the excavations to mode selection in BA-QCLs may beuseful, even the example experimental data provided herein shows thatonly a small perturbation can cause large changes in mode selection.Therefore, the high order transverse modes and the fundamental mode maybe very nearly competitive with each other in BA-QCLs, where thedependence of threshold gain with mode number is somewhat weak in theunperturbed device. As a result, a vast array of geometries may have asimilar effect.

Therefore, as described herein, and in particular as demonstrated in theexample experiments described above, large changes in the transversemode selection in BA-QCL devices may be induced by introducing pairs ofexcavations, e.g., generated by focused ion beam milling, in smallportions of the BA-QCL device. The proximity of the excavations andtheir depth may have a critical influence, and if selected properly, theexcavations may extract most of the emitted power from the fundamentalmode that was not favored in an unaltered BA-QCL device. Furtheroptimization of the excavation geometry may produce an even betterbalance between a purer fundamental mode and change in threshold currentand slope efficiency. Once optimized, the formation of excavations maybe easily incorporated into a photolithographic fabrication process orthe like.

FIG. 9 is a flow chart of a method for extracting fundamental transversemode operation in a BA-QCL, in accordance with a representativeembodiment.

As shown in block 902, the method 900 may include selecting propertiesfor a perturbation (e.g., an excavation) to be created in the opticalcavity of a BA-QCL device. For example, this may include selecting atleast one of a size, a shape, and a depth of one or more excavations toalter the lateral refractive index profile of the optical cavity asperceived by light present in the optical cavity of a BA-QCL. This mayalso or instead include selecting a number of excavations to include inthe optical cavity of the BA-QCL.

As shown in block 904, the method 900 may include forming a perturbationin an optical cavity of a BA-QCL, where the perturbation extends fromone or more walls of the optical cavity. As discussed herein, theperturbation may include a plurality of excavations. Thus, block 904 mayinclude forming at least two excavations in a top surface (or anothersurface) of an optical cavity of a BA-QCL. The optical cavity mayinclude an active region disposed between a top cladding and a bottomcladding, where the excavations extend into at least the top cladding ofthe optical cavity. Stated otherwise, block 904 may include forming atleast two excavations in the top surface of the optical cavity of aBA-QCL, where the excavations are structurally configured to select afundamental transverse mode of light in the optical cavity byconstricting a lateral refractive index profile of the optical cavity.Block 904 may also or instead include providing other forms ofperturbations in the optical cavity.

As shown in block 906, the method 900 may include extending one or moreof the excavations into the active region of the optical cavity. Thismay also or instead include extending one or more of the excavationsinto the bottom cladding of the optical cavity.

The method 900 may also include, e.g., as part of the processes of block904 or block 906 described above, selecting at least one or a size and ashape of a perturbation to modify a lateral refractive index profile ofthe optical cavity. For example, where the perturbation includes one ormore excavations, each of the excavations may include a shape and a sizeselected such that the excavations collectively modify a lateralrefractive index profile of the optical cavity.

As shown in block 908, the method 900 may include altering a lateralrefractive index profile of the optical cavity, e.g., constricting thelateral refractive index profile of the optical cavity. The lateralrefractive index profile of the optical cavity may be altered by theformation and presence of the perturbation (e.g., one or moreexcavations).

As shown in block 910, the method 900 providing a loss to high ordertransverse optical modes in the optical cavity using a perturbation.Thus, where the perturbation includes one or more excavations, themethod 300 may include providing a loss to high order transverse modesof light in the optical cavity using the excavations. In other words,the excavations may provide additional loss to spatially filter the highorder transverse modes of light in the optical cavity. Stated otherwise,any perturbation formed in the optical cavity may suppress high ordertransverse optical modes during operation of the broad area quantumcascade laser, whereby a fundamental transverse optical mode is selectedover the high order transverse optical modes.

As shown in block 912, the method 900 may include selecting afundamental transverse mode of light in the optical cavity using theperturbation (e.g., the one or more excavations).

The light emitted from the optical cavity may include one or more laserbeams. For example, as shown in block 914, the method 900 may includeproviding a primarily single-lobed laser beam emitting from the opticalcavity. The single-lobed laser beam may be formed at least in part bythe excavations. That is, in certain implementations, without theexcavations, the single-lobed laser beam would not be emitted from theoptical cavity, e.g., multiple laser beams would instead be emitted fromthe optical cavity. Stated otherwise, the method 900 may includeemitting a laser beam having a single lobe aligned with an optical axisof the optical cavity, where the plurality of excavations arestructurally configured to collectively select the fundamentaltransverse optical mode over the high order transverse optical modes toprovide the laser beam having the single lobe.

As shown in block 916, the method 900 may include providing apredetermined brightness of light emitted from the optical cavity. Thismay also or instead include providing one or more other predeterminedproperties of light emitted from the optical cavity including withoutlimitation power, intensity, luminosity, flux, wavelength, frequency,and so on.

Thus, devices, systems, and methods disclosed herein may include theimplementation of a relatively small, localized change in the lateralindex profile of an optical cavity that is able to extract fundamentaltransverse mode operation in a BA-QCL device, which results in enhancedbrightness and advantageous operability of the laser beam. In otherwords, a BA-QCL may be modified by placing a local perturbation in thelateral refractive index profile in a manner that selectively favorslasing in the transverse fundamental mode, when it would not otherwisebe favored. The devices, systems, and methods disclosed herein may betranslatable to all commonly practiced semiconductor fabricationmethods, and may result in the scaling of brightness in BA-QCLs.

As stated above, although some of the accompanying figures showrectangular excavations (e.g., formed by focused ion beam milling), evenwhen excavations are formed in a different manner, or have differentgeometries, or are filled with different materials, the techniques maycontinue to provide transverse mode selectivity sufficient to extractthe fundamental mode and enhance a laser device.

A disclosed apparatus may include a BA-QCL that is capable of emitting asingle lobed beam along the optical axis as a result of fundamentaltransverse mode operation that is made possible by the placement of alocal perturbation in the lateral refractive index profile. For example,the local lateral refractive index profile may be achieved by etchingexcavations or trenches in the BA-QCL device. The local refractive indexprofile may also or instead be achieved by alternative methods, such asion implantation, or selective current injection. Plasma etching methodsthat provide a local lateral refractive index may be formed byalternative means, including, but not limited to, focused ion beammilling. The local index profile may be achieved using excavationsand/or other features with different shapes and geometries. Theexcavations formed (to provide the lateral index profile) may befilled-in with other materials, such as metals or oxides, or they may becompletely devoid of material. A local index restriction may be used toselect the fundamental mode in any type of laser cavity where thefundamental mode operation is not otherwise favored.

Unlike prior art techniques of using ion milled lateral constriction ina waveguide having a narrow cavity, in this disclosure, lateralconstrictions in the waveguide are used in BA-QCLs with a much largercavity width (w≈90 μm), with an eye toward promoting the fundamentalmode in these devices so that the brightness can be substantiallyincreased.

It will be appreciated that, although the devices, systems, and methodsdescribed above generally reference use in a BA-QCL, other broad areasemiconductor diode lasers that are not QCLs may also or instead utilizethe devices, systems, and methods described herein. In other words, thedevices, systems, and methods described herein may be used in otherbroad area semiconductor diode lasers where high-order transverse modespreclude the emission of a single lobed beam originating from thefundamental mode.

It will be appreciated that the devices, systems, and methods describedabove are set forth by way of example and not of limitation. Absent anexplicit indication to the contrary, the disclosed steps may bemodified, supplemented, omitted, and/or re-ordered without departingfrom the scope of this disclosure. Numerous variations, additions,omissions, and other modifications will be apparent to one of ordinaryskill in the art. In addition, the order or presentation of method stepsin the description and drawings above is not intended to require thisorder of performing the recited steps unless a particular order isexpressly required or otherwise clear from the context.

The method steps of the implementations described herein are intended toinclude any suitable method of causing such method steps to beperformed, consistent with the patentability of the following claims,unless a different meaning is expressly provided or otherwise clear fromthe context. So, for example performing the step of X includes anysuitable method for causing another party such as a remote user, aremote processing resource (e.g., a server or cloud computer) or amachine to perform the step of X. Similarly, performing steps X, Y, andZ may include any method of directing or controlling any combination ofsuch other individuals or resources to perform steps X, Y, and Z toobtain the benefit of such steps. Thus, method steps of theimplementations described herein are intended to include any suitablemethod of causing one or more other parties or entities to perform thesteps, consistent with the patentability of the following claims, unlessa different meaning is expressly provided or otherwise clear from thecontext. Such parties or entities need not be under the direction orcontrol of any other party or entity, and need not be located within aparticular jurisdiction.

It should further be appreciated that the methods above are provided byway of example. Absent an explicit indication to the contrary, thedisclosed steps may be modified, supplemented, omitted, and/orre-ordered without departing from the scope of this disclosure.

It will be appreciated that the methods and systems described above areset forth by way of example and not of limitation. Numerous variations,additions, omissions, and other modifications will be apparent to one ofordinary skill in the art. In addition, the order or presentation ofmethod steps in the description and drawings above is not intended torequire this order of performing the recited steps unless a particularorder is expressly required or otherwise clear from the context. Thus,while particular embodiments have been shown and described, it will beapparent to those skilled in the art that various changes andmodifications in form and details may be made therein without departingfrom the scope of this disclosure and are intended to form a part of thedisclosure as defined by the following claims, which are to beinterpreted in the broadest sense allowable by law.

The various representative embodiments, which have been described indetail herein, have been presented by way of example and not by way oflimitation. It will be understood by those skilled in the art thatvarious changes may be made in the form and details of the describedembodiments resulting in equivalent embodiments that remain within thescope of the appended claims.

What is claimed is:
 1. A broad area quantum cascade laser, comprising:an optical cavity comprising an active region disposed between a topcladding and a bottom cladding; at least two excavations being formed ina top surface of the optical cavity, the at least two excavationsprotruding into the top cladding of the optical cavity, and beyond thetop cladding and into the bottom cladding, the at least two excavationslocally reducing a refractive index of the optical cavity; one or moreof the at least two excavations having a cross section comprised of apolygon, a rectangle, a circle, an ellipse or an oblong shape, and aportion of the optical cavity disposed at least partially between the atleast two excavations, where, during operation of the broad area quantumcascade laser, the at least two excavations favor a fundamentaltransverse mode of light within the portion by suppressing high ordertransverse optical modes of light.
 2. The broad area quantum cascadelaser of claim 1, wherein the fundamental transverse mode of light inthe optical cavity would not be favored in the absence of the at leasttwo excavations.
 3. The broad area quantum cascade laser of claim 1,wherein the portion of the optical cavity in which the fundamentaltransverse mode of light is favored comprises an entire width and lengthof the optical cavity.
 4. The broad area quantum cascade laser of claim1, wherein the optical cavity is fabricated from a structure grown bygas source molecular beam epitaxy on an n− InP(001) substrate.
 5. Thebroad area quantum cascade laser of claim 1, wherein each of the atleast two excavations define a void that lacks material therein.
 6. Thebroad area quantum cascade laser of claim 1, wherein the locally reducedrefractive index of the optical cavity established by the at least twoexcavations is a function of a geometry of the at least two excavations.7. The broad area quantum cascade laser of claim 1, wherein the topsurface comprises the top cladding.
 8. The broad area quantum cascadelaser of claim 1, wherein the at least two excavations are structurallyconfigured to one or more of provide loss to the high order transversemodes of light and cause the broad area quantum cascade laser to emit asingle-lobed laser beam.
 9. The broad area quantum cascade laser ofclaim 1, wherein at least one of a size, a shape, and a depth of each ofthe at least two excavations is selected to alter a lateral refractiveindex profile of the optical cavity to affect mode selection that isspecific to light generated by the broad area quantum cascade laser. 10.The broad area quantum cascade laser of claim 1, wherein a size and ashape of each of the at least two excavations are identical.
 11. Thebroad area quantum cascade laser of claim 1, wherein the at least twoexcavations are substantially aligned along an axis substantiallyperpendicular to an optical axis of the optical cavity.
 12. A broad areaquantum cascade laser, comprising: an optical cavity comprising anactive region disposed between a top cladding and a bottom cladding; atleast two excavations being formed in a top surface of the opticalcavity and protruding into at least the top cladding, the at least twoexcavations locally reducing a refractive index of the optical cavity;the optical cavity approximating a box having a geometry defined by aheight of the optical cavity, a width of the optical cavity defined bysidewalls of the optical cavity, and a length of the optical cavity; thesidewalls being etched to a depth below that of the active region; and aportion of the optical cavity being disposed at least partially betweenthe at least two excavations, where, during operation of the broad areaquantum laser, the at least two excavations favor a fundamentaltransverse mode of light within the portion by suppressing high ordertransverse optical modes of light.
 13. A method, comprising: forming atleast two excavations in a top surface of an optical cavity of a broadarea quantum cascade laser, the optical cavity comprising an activeregion disposed between a top cladding and a bottom cladding, the atleast two excavations extending into the top cladding of the opticalcavity, and beyond the top cladding and into the bottom cladding; one ormore of the at least two excavations having a cross section comprised ofa polygon, a rectangle, a circle, an ellipse or an oblong shape; locallyreducing a refractive index of the optical cavity with the at least twoexcavations; suppressing high order transverse optical modes of lightwithin the optical cavity with the at least two excavations; andfavoring a fundamental transverse mode of light within a portion of theoptical cavity disposed at least partially between the at least twoexcavations.
 14. The method of claim 13, further comprising providing aloss to high order transverse modes of light in the optical cavity usingthe at least two excavations.
 15. The method of claim 13, wherein eachof the at least two excavations define a void that lacks materialtherein.
 16. The method of claim 13, further comprising selecting ageometry of the at least two excavations to provide a predeterminedlocally reduced refractive index of the optical cavity.